The utility and advantage of aqueous ammonium polyphosphate are well known to the agricultural industry. The benefits of high acyclic polyphosphate levels are also recognized. It is often desirable, if not essential, to use merchant grade, wet-process acid feeds due to availability and cost. Wet-process acids are obtained by acidifying phosphate rocks, usually containing calcium phosphates, with strong mineral acids, such as sulfuric acid, which convert the calcium to calcium sulfate releasing phosphoric acid. Insoluble calcium sulfate is separated by filtration.
The merchant grade acid thus obtained is relatively dilute, usually containing less than 55 percent P.sub.2 O.sub.5. The solutions also contain numerous cogeneric metallic impurities extracted from the source rock including compounds of iron, calcium, aluminum and magnesium. Direct ammoniation of the dilute acids forms unstable ammonium phosphate solutions of low P.sub.2 O.sub.5 content from which the metallic impurities precipitate, rendering the solutions essentially useless in many applications. This precipitation problem can be substantial in view of the high impurity content of some merchant grade acids.
The ability of polyphosphates to chelate these impurities or otherwise prevent their precipitation is known. It remains, however, to devise an economical process for converting wet-process acids to concentrated ammonium phosphate solutions containing sufficient polyphosphates to prevent precipitation throughout the life of the product.
While long shelf life, i.e., solution stability, is always desirable, it is sometimes essential. For instance, the demand for ammonium polyphosphate, particularly in the agricultural industry, is highly seasonal. As much as 95 percent of the total annual deliveries for some dealers is required within a six-week period, generally during the spring. Obviously, it is very undesirable for such dealers to invest the capital in on-site equipment required to produce the total annual demand in that six-week period. Yet that is precisely what would be required if the stability of the ammonium phosphate product is not sufficient to allow long storage periods. A process capable of substantially improving solution stability would markedly reduce unit size by allowing the operator to run over a greater part of the year and would also increase his flexibility responsive to demand. Moreover, solutions having higher polymeric phosphate concentrations (relative to the total P.sub.2 O.sub.5 content) are also more able to dissolve and retain even pure solute whereby higher concentrations can be employed. The costs of handling, storing and transporting the product is reduced commensurately. Numerous attempts have been made at developing such a process.
Earlier efforts were directed to the formation of concentrated acid solutions from merchant grade acids by gradual evaporation. Treatment at sufficiently high temperatures produces polyphosphoric acid which, when neutralized under appropriate conditions, forms ammonium polyphosphates. The polymeric species serve to prevent impurity precipitation throughout the process. However, multiple process steps and high heat loads detract from overall economics.
Much effort was made to perfect the direct ammoniation of merchant grade acids taking advantage of exothermic neutralization which can generate sufficient heat to drive water from the liquid phase and produce polyphosphates. Investigators pursuing this approach found that conditions sufficient to obtain high polyphosphate levels also produce cyclic metaphosphates that accumulate as fouling deposits on hot contact surfaces. One approach involves the so-called pipe or jet reactors in which the acid and ammonia are contacted in a confined tubular reaction zone under conditions sufficient to concentrate and polymerize the phosphate. Temperatures on the order of 500.degree. F. and up have been employed for this purpose. The published results of many of these endeavors indicate that the deposit accumulation rate on interior reactor walls is so great that such length and process economics suffer substantially. Fouling rate increases with reaction temperature and can become so severe that continuous operations can not be maintained for more than two hours without removing and re-working the reactor tube.
The severity of the problem and continuing interest in its solution are evidenced by more recent work on reactor fouling. For instance, fouling prevention by addition of urea to the reactor feed is suggested in U.S. Pat. No. 3,723,086. Other investigators have suggested the use of cold wall reactors believing that lower wall temperature would reduce deposit accumulation. These efforts have been successful to varying degrees. However, reactor wall cooling also reduces reaction temperatures which, in turn, limit polyphosphate content.
Moreover, the question of reactor fouling is not a one-sided issue. In fact, most previous investigators readily discovered that the accumulation of fouling deposits on the reactor interior is their only weapon against extremely rapid corrosion. Phosphoric acids, even at the concentrations found in merchant grade acids, corrode and dissolve essentially all known metals and alloys as well as glass and ceramics often used as reactor liners at temperatures above 500.degree. F. Yet, as described hereinafter, those temperatures must be obtained to produce significant polyphosphate levels. Thus, presently available processes, in order to operate effectively for any significant period, would have to control the reactor fouling rate at a level such that corrosion is avoided while excessive reactor fouling and plugging do not occur. Processes capable of maintaining that balance are not available.
Moreover, this benefit of reactor fouling is not even available with the more pure "white acids". The white acids contain at most insignificant amounts of foulant producing materials. The available batch or tubular reactors can not operate on white acid feeds for this reason.
The so-called white acids are obtained in the "electric furnace" process in which phosphate-containing rock is reduced by reaction with coke at extremely high temperatures generated by electric current. The phosphate is reduced to elemental phosphorus, burned to P.sub.2 O.sub.5 and absorbed in water. While these acids are generally more expensive than those obtained from the wet-acid process, they often become available at prices low enough to justify their use in fertilizer manufacture due to regional over production. However, for the reasons mentioned above, these acids are not suitable for tubular or batch neutralization. They do not form deposits sufficient to protect reactor internals with the consequence that corrosion rates are exceedingly high. Our process allows the use of any acid feed, regardless of impurity level, including wet-process, white, or combinations of these or other acids.
As discussed hereinafter in more detail, maximum polymer content is limited essentially by maximum reactor temperature, assuming equilibrium is obtained. Maximum temperature, in turn, is limited by the feed water content, i.e., the H.sub.2 O/P.sub.2 O.sub.5 ratio. Thus, while reactor wall cooling may, to some extent, eliminate the problem of reactor fouling, it also reduces product quality.
It is therefore one object of this invention to provide a method of producing stable ammonium phosphate solutions from either wet-process or white acids. Another object is the provision of an improved method for the direct neutralization of these acids with ammonia which makes maximum use of the autogenous heat of neutralization. Another object is the conversion of merchant grade wet-process acids containing cogeneric metallic impurities to stable aqueous ammonium phosphate solutions in which at least about 40, and preferably at least about 50 percent of the phosphorous is present as acyclic polyphosphates. Another object is the provision of a method for the production of stable, concentrated ammonium phosphate solutions in which at least 40 percent of the phosphorous occurs as non-orthophosphoric species from relatively pure white acid feeds while minimizing the reactor corrosion.
Therefore, in accordance with one embodiment, phosphoric acids having H.sub.2 O/P.sub.2 O.sub.5 mole ratios of about 4.5 or less are converted to stable, aqueous ammonium polyphosphate solutions of which at least about 40 percent of the phosphorous determined as P.sub.2 O.sub.5 is present as acyclic polyphosphates. This is accomplished by spraying the acid downwardly into an unconfined reaction zone to produce a highly dispersed, discontinuous liquid acid phase, and reacting substantially anhydrous ammonia with the acid spray. Reaction conditions are controlled to obtain a liquid phase temperature of about 500.degree. to about 750.degree. F. The reacted acid spray is then quenched to a temperature below about 200.degree. F. in an ammonium polyphosphate solution.
The unconfined reaction zone reduces or completely eliminates the effects of fouling deposit accumulation on reactor internals. It is advisable to surround the entire reaction system in a substantially gas tight vapor housing of relatively large internal diameter to prevent air induction and escape of ammonia, acid and reaction products. Nevertheless, the reaction takes place almost exclusively, is not exclusively, in an unconfined, highly dispersed, discontinuous liquid phase. This is accomplished by producing a dispersed liquid spray within a relatively large chamber and directing the spray downwardly into the quench zone such that 20 percent or less of the acid medium contacts the container walls. Thus, heat transfer through the reactor walls does not account for any substantial heat loss as regards a reaction medium per se, i.e., the liquid acid droplets. The housing walls remain relatively cool compared to the hot acid reaction phase and are contacted with little if any of the liquid phase. Foulant accumulation is minimized or eliminated. Corrosion rate is significantly reduced, even with non-fouling "white" acids due to low wall temperatures on the order of 400.degree. F. or less. Reduced wall temperatures do not detract from conversion level in this system since the required conversion to polymer has already occurred in the liquid phase before contact, if any, with the reactor walls. We have discovered that extremely high reaction rates can be obtained in highly dispersed acid phase by this procedure rates capable of producing the required conversion in a very short time span, less than that required for the acid spray to travel either to the vapor housing or quench.
Some investigators have suggested that the degree of some problems associated with tubular reactors might be reduced by injecting ammonia at high mass rates and/or velocities. While this remedy might have some benefits, it introduces additional complications due primarily to the vast dissimilarity in the physical properties of the two phases -- acid and ammonia. Even at the elevated temperatures required in this reaction, the acid phase, and the ammonium phosphate or polyphosphate melt, are extremely viscous. Accordingly, ammonia injection at high velocities results in two separated, yet continuous phases, with the acid phase clinging to the flowing along the interior reactor walls while the ammonia gas passes directly through the tube. In straight reactor tubes this phenomenon results in low ammonia conversions and incomplete acid conversion. Very little acid surface is exposed to the ammonia gas. Secondly, the acid or partially neutralized melt at the acid-gas interface, may be subject to high shear, and may even be literally blown out of the reactor before complete conversion can take place.
We considered that the formation of two discrete continuous phases might be overcome with a reactor tube having one or more sharp angles or bends, or one provided with baffles to promote acid-ammonia mixing. Very little if any mixing would result from this procedure due to the vast difference in viscosity of the liquid and gas phases. Moreover, this approach would definitely result in increased reactor fouling and/or corrosion due to exposure of even higher internal surface (baffles), or erosion or protective foulant deposits from the angular parts of the reactor tube.
For instance, this type of erosion-corrosion is known to result in the ordinary pipeline transport of phosphoric acids due to the removal of protective phosphate film in high turbulence zones. However, in the absence of such turbulence in a tubular reactor, low conversions of both ammonia and acid will result without relatively long contact times, e.g., long reactor tubes. The gas phase will pass quickly through the reactor while the acid, travelling in a continuous liquid phase is subject to little radial mixing due to its extremely high viscosity. In fact, the acid at the acid-reactor interface may never come into direct contact with the ammonia gas.
The continued attempts to perfect confined liquid phase reaction systems, such as pipe reactors, is understandable in view of the belief that the reaction should be contained in a relatively small volume to promote adequate mixing and retain the heat required to produce the high temperatures necessary for high conversion to polyphosphates. It was not apparent that these temperatures could be obtained in a highly dispersed, discontinuous liquid phase. Under these circumstances the reaction phase -- the liquid spray -- presents such a high external surface area that heat exchange between the reaction phase and ambient ammonia gas would increase, thereby lowering reaction temperature. However, while heat transfer rate undoubtedly increases with surface, we have found that the required temperatures can still be obtained.
The reasons for the success of this procedure are not known with certainty, although it is possible that the reaction rate is increased sufficiently by the exposure of the higher surface area to provide reaction rates within the acid droplets high enough to generate heat at a rate greater than it is transferred to the immediate environment, thereby allowing attainment of the required temperatures. The attainment of high temperatures in the acid droplets may involve the formation of a heat transfer barrier or inhibiting film at the drop surface due to characteristics of either the original acid, the reaction products or some intermediate form thereof.
However, it is also apparent from our successful operations that the presence of some type of heat transfer barrier at the droplet periphery, if it does occur, does not reduce the ammonia transfer rate to an extent sufficient to prevent the high neutralization rates and temperatures required for polymerization.
High ammonia transfer rates must be maintained to obtain sufficient liquid phase temperature while avoiding excessive heat loss of the liquid droplets to the environment. A critical factor in this regard is the H.sub.2 O/P.sub.2 O.sub.5 ratio of the feed as it enters the reaction zone. Substantially anhydrous ammonia is also required. The presence of water in the ammonia gas influences reaction kinetics and conversion in a manner essentially identical to variation of the H.sub.2 O/P.sub.2 O.sub.5 feed ratio. Thus, the amount of water contained in the injected ammonia gas should also be taken into account in determining that ratio.
It was not apparent that parameters could be found or adequately controlled to maintain this essential balance. For instance, Y. A. K. Abdul-Rahman and E. J. Crosby observed the formation of an impervious crust upon treatment of phosphoric acid droplets with ammonia. Their results are reported in "Direct Formation of Particles from Drops by Chemical Reaction With Gases", Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin, appearing in Chemical Engineering Science (1973), Vol. 28, pages 1273-1284. The efforts of these authors, as indicated by the title of their work, was directed to the formation of solid droplets of ammonium phosphate. While their investigations were carried out at temperatures far below those required to promote polymerization, and at contact times far exceeding those possible in apparatus of the type described herein, they did observe several phenomena which appeared to negate the utility of this process. These included the rapid formation of a crust surrounding the droplets, the attainment of only very low temperatures even with anhydrous ammonia, and the actual explosion of the droplets in some cases due to containment of vaporized water by the ammonium phosphate crust.
We have discovered that the formation of such mass transfer barriers must, and can be prevented. However, the prevention of that phenomenon requires that the reaction rate within the droplets be sufficiently high to reach temperatures in excess of the ammonium phosphate melting point prior to the occurence of any significant crust formation at the droplet's periphery. This can be accomplished in our process. While a number of variables are doubtlessly involved to different extents, the most significant factors are probably the H.sub.2 O/P.sub.2 O.sub.5 feed ratio and adequate liquid phase dispersion. If the ratio of water to phosphate exceeds the maximum of 4.5 H.sub.2 O/P.sub.2 O.sub.5, more preferably 3.3, droplet temperature can not be elevated to the point required to obtain in excess of 40 percent polymeric species in the product, and can not be elevated at a rate sufficient to reach the ammonium phosphate melting point prior to the formation of crusts at the particle periphery. Finally, assuming that temperatures above the water boiling point could be achieved in these more dilute systems, the pressure buildup due to the generation of the steam in the particle interior would result in the particle fragmentation as observed by Abdul-Rahman et al.
While numerous other embodiments and variations of these systems will be apparent from the principles disclosed, one embodiment bears particular mention. The necessity of producing a highly dispersed acid phase has been pointed out. However, the degree of dispersion obtainable by any given spray means is at least in part a function of feed acid viscosity. Thus, as the viscosity of the feed increases, the degree of dispersion is reduced. Upon observing this occurrence and its effect on controlling parameters, we discovered that adequate dispersion can be maintained even with highly viscous acid feeds without equivalent fouling by heating the feed to a temperature between about 250.degree. and about 600.degree. F. With wet-process feeds, temperatures substantially above this level should be avoided to prevent cyclic metaphosphate production and apparatus fouling upstream of the reactor or within the acid spray means itself. While there is, of course, no precise feed acid viscosity above which feed preheating should be employed, this embodiment is particularly beneficial for use with feeds having viscosities in excess of about 4000 centipoise at 80.degree. F.